The Landers earthquake, which had a moment magnitude (M(w)) of 7.3, was the largest earthquake to strike the contiguous United States in 40 years. This earthquake resulted from the rupture of five major and many minor right-lateral faults near the southern end of the eastern California shear zone, just north of the San Andreas fault. Its M(w) 6.1 preshock and M(w) 6.2 aftershock had their own aftershocks and foreshocks. Surficial geological observations are consistent with local and far-field seismologic observations of the earthquake. Large surficial offsets (as great as 6 meters) and a relatively short rupture length (85 kilometers) are consistent with seismological calculations of a high stress drop (200 bars), which is in turn consistent with an apparently long recurrence interval for these faults.

We present a new three-dimensional model of the major fault systems in southern California. The model describes the San Andreas fault and associated strikeslip fault systems in the eastern California shear zone and Peninsular Ranges, as well as active blind-thrust and reverse faults in the Los Angeles basin and Transverse Ranges. The model consists of triangulated surface representations (t-surfs) of more than 140 active faults that are defined based on surfaces traces, seismicity, seismic reflection profiles, wells, and geologic cross sections and models. The majority of earthquakes, and more than 95% of the regional seismic moment release, occur along faults represented in the model. This suggests that the model describes a comprehensive set of major earthquake sources in the region. The model serves the Southern California Earthquake Center (SCEC) as a unified resource for physics-based fault systems modeling, strong ground-motion prediction, and probabilistic seismic hazards assessment.

Paleoseismic data from 11 trenches at seven sites excavated across the southern Johnson Valley, Kickapoo, and Homestead Valley faults that ruptured in the 1992 Landers earthquake, as well as the northern Johnson Valley fault which did not fail in 1992, indicate that the return period for large surface rupturing events in this part of the eastern California shear zone is in the range of 5-15 ka. The inferred slip rates, based on their respective recurrence intervals, are in the range of 0.2-0.6 mm/yr for each of the faults studied. A previous large earthquake ruptured the southern Johnson Valley and Kickapoo faults about 5 ka B.P. The northern Johnson Valley fault also failed at about this time at 5.8 ka B.P. and may have been part of the same rupture. In contrast, the penultimate large earthquake that we identify on the Homestead Valley fault occurred about 15 ka B.P., much earlier than other faults involved in the 1992 rupture. From these observations, combined with paleoseismic work by others after the 1992 earthquake, it appears that previous events along the southern Johnson Valley and Kickapoo faults were different than those of 1992 and may have involved other fault segments. It has been over 5 ka since the most recent rupture on the northern Johnson Valley fault. Therefore, it is surprising that it did not fail in the 1992 rupture. From our observations, dextral shear appears to be distributed across the entire eastern California shear zone, with individual faults taking only a small proportion of the overall slip. Release of this regional strain appears to occur in temporal clusters of large (?) earthquakes, with the 1992 event apparently the most recent of a sequence of late Holocene (0-1 ka) earthquakes that have ruptured the nine faults we have trenched in the southwestern Mojave desert. Previous clusters of earthquake activity occurred in the early (8-9 ka) and middle (5-6 ka) Holocene, and possibly the latest Pleistocene (ϳ15 ka).

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The Rose Canyon fault zone in San Diego, California, has many well‐expressed geomorphic characteristics of an active strike‐slip fault, including scarps, offset and deflected drainages and channel walls, pressure ridges, a closed depression, and vegetation lineaments. Geomorphic expression of the fault zone from Mount Soledad south to Mission Bay indicates that the Mount Soledad strand is the most active. A network of trenches excavated across the Mount Soledad strand in Rose Creek demonstrate a minimum of 8.7 m of dextral slip in a distinctive early to middle Holocene gravel‐filled channel that crosses the fault zone. The gravel‐filled channel was preserved within and east of the fault but was removed west of the fault zone by erosion or possibly grading during development. Consequently, the actual displacement of the channel could be greater than 8.7 m. Radiocarbon dates on detrital charcoal recovered from the sediments beneath the channel yield a maximum calibrated age of about 8.1±0.2 kyr. The minimum amount of slip along with the maximum age yield a minimum slip rate of 1.07±0.03 mm/yr on this strand of the Rose Canyon fault zone for much of Holocene time. Other strands of the Rose Canyon fault zone, which are east and west of our site, may also have Holocene activity. Based on an analysis of the geomorphology of fault traces within the Rose Canyon fault zone, along with the results of our trenching study, we estimate the maximum likely slip rate at about 2 mm/yr and a best estimate of about 1.5 mm/yr. Stratigraphie evidence of at least three events is present during the past 8.1 kyr. The most recent surface rupture displaces the modern A horizon (topsoil), suggesting that this event probably occurred within the past 500 years. Stratigraphie and structural relationships also indicate the occurrence of a scarp‐forming event at about 8.1 kyr, prior to deposition of the gravel‐filled channel that was used as a piercing line. A third event is indicated by the presence of several fault strands that displace the channel but did not move during the most recent event. Other events may also have occurred, but these data suggest that the return time for surface‐rupturing earthquakes is no more than about 4 kyr.